Circuit integrity in a packet-switched network

ABSTRACT

Techniques for performing a continuity check operation include sending a pattern of bits over a packet network connection through a first interface on a packet network to a second interface on the packet network. The first interface is monitored for return of the pattern of bits over the packet network connection. A decision whether the continuity check is successful is based on whether the pattern of bits is detected at the first interface during the monitoring. The techniques can be used for both narrowband as well as broadband calls over the packet network.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority of U.S.Provisional Patent Application No. 60/147,462, filed Aug. 6, 1999, andincorporated herein by reference.

The following U.S. patent applications, filed concurrently with thisapplication and assigned to the assignee of this application, areincorporated herein by reference: (1) “Communications Using HybridCircuit-Switched and Packet-Switched Networks,” Ser. No. 09/633,523,filed Aug. 4, 2000 and (2) “Bandwidth Management in a CommunicationsSystem Using Circuit-Switched and Packet-Switched Networks,” Ser. No.09/633,524, filed Aug. 6, 2000.

BACKGROUND

The invention relates to circuit integrity in a packet-switched network.

System Signal 7 (SS7) messages are often used to provide control signalsin various telecommunications systems, such as telephone systems, andprovide a mechanism, known as continuity check, for checking theintegrity of a circuit between two switching network endpoints duringcall setup. Continuity checks originally were developed for analogfacilities and consist, for example, of a frequency tone transmitted bythe originating exchange and looped back by the receiving exchange.Reception of the returned tone by the originating exchange indicatesthat the channel is available. In digital environments, use ofcontinuity check operations has been similar.

Recently, packet-domain network architectures, such as asynchronoustransfer mode (ATM) networks, have been considered for transportingvoice and other narrowband traffic. Packet networks allow connections tobe made between endpoints without dedicated inter-switch connections.Fixed-size packets of data, known as cells, are transferred between theATM switches, which are packet switches that provide virtual circuitsbetween the end points of a network.

With the advent of packet voice networks and the introduction ofadaptation between circuit-switched and packet-switched bearers,high-quality integrity checks should be capable of detecting andisolating various types of failures. Traditional testing by continuitycheck tones, however, is incapable of detecting certain failures suchas, for example, impaired bits having low significance with respect to atime-domain multiplexed (TDM) sample value. Therefore, better-qualitycontinuity checking suitable to packet networks is desirable.

SUMMARY

In general, according to one aspect, techniques for performing acontinuity check operation include sending a pattern of bits over apacket network connection through a first interface on a packet-switchednetwork to a second interface on the packet-switched network. The firstinterface is monitored for return of the pattern of bits over the packetnetwork connection. A decision whether the continuity check issuccessful is based on whether the pattern of bits is detected at thefirst interface during the monitoring.

In various implementations, one or more of the following features may bepresent. The technique can be used in connection with both narrowbandand broadband communications. The continuity check can be performed, forexample, during a set-up process for a narrowband call over a packetnetwork. The call set-up process can include sending Signaling System 7(SS7) messages. The techniques can include providing a loop thatconnects incoming and outgoing packet streams associated with the packetnetwork connection. The particular pattern of bits may vary depending onvarious factors including the type of failures to be detected. Thepattern of bits can be sent repeatedly over the packet networkconnection during the monitoring.

A system for performing continuity check operations for traffic that isto be transported over a packet network also is disclosed. The systemcan include a first gateway that is coupled to a first interface on thepacket network and that is configured to execute continuity checkoperations. The gateway includes a bit pattern generator arranged togenerate a pattern of bits to be sent over a packet network connection,and a bit pattern detector arranged to monitor return of the pattern ofbits over the packet network connection. The gateway is configured todecide whether a continuity check is successful based on whether thegenerated pattern of bits is detected by the bit pattern detector.

The system also can include a second gateway coupled to a secondinterface on the packet network and configured to provide a loop betweenincoming and outgoing packet streams associated with the packet networkconnection. In some implementations, the gateways may be configured toadapt circuit-switched and packet-switched bearers.

Various implementations may include one or more of the followingadvantages. The techniques described here can help ensure thenon-disruptable transfer of data through a packet-switched network. Inparticular, continuity check operations employing a pattern of bits canbe used to test for and isolate a wide range of potential failures thatmay occur in a packet-domain connection or the adapters terminating thepath. Therefore, the continuity checks can help ensure that the circuitwill faithfully reproduce the continuity check operation can reduce thenumber of resources required to execute the test compared to frequencyor tone-based continuity check operations.

Other features and advantages will be readily apparent from thefollowing detailed description, the accompanying drawings and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a telephone connection through a hybrid ATMnetwork and an associated signaling network.

FIG. 2 is a simplified block diagram of an exemplary media gateway.

FIGS. 3A and 3B are a flow chart of a method for employing continuitychecks in a voice call set up process over an ATM network.

FIG. 4 is a signal flow diagram corresponding to FIG. 3.

FIG. 5 illustrates further details for performing a continuity checkoperation.

FIG. 6 illustrates an exemplary bit pattern.

DETAILED DESCRIPTION

As discussed in greater detail below, improved techniques are describedfor providing continuity check operations to ensure circuit integrityfor communications over a packet-switched network. The techniquesinvolve the exchange of a known pattern of bits during call set upprocesses rather than sending and detecting tones. The techniques can beused to test the integrity of the packet circuit and the adapters ateither end of the circuit.

The particular examples discussed below involve narrowband traffic suchas voice calls, modem data or facsimile data, sent over apacket-switched network. However, continuity checks that include apattern of bits can be used for broadband communications over apacket-switched network as well.

As shown in FIG. 1, a continuous call path can be established startingwith a narrowband Signaling System 7 (SS7) ISDN user part (ISUP) callthat originates, for example, in a Public Switched Telephone Network(PSTN) 102A. The path can be established using a virtual circuit over anATM network 101 and completes on the terminating side in a narrowbandcircuit-switched SS7 ISUP call to the terminating subscriber throughanother circuit switched network 102B. A control mechanism interactswith the circuit-switched and packet-switched networks to correlate SS7and ATM connections to establish a single continuous information path.

A large number of individual telephone circuits, such as DS0 circuits,that are to be connected to the packet network 101 can be carried, forexample, on fiber optic carriers 105 using time-division multiplexing(TDM) according to the Telcordia Synchronous Optical Network (SONET)standards. The carriers 105 are coupled to access ports 116 in mediagateways 100A, 100B (see FIG. 2).

The media gateways 100A, 100B adapt the TDM telephone line signals topacket-based signals and vice-versa. The TDM telephone signals arecircuit-switched, in other words, the bit stream can be dividedtemporally into individual DS0 circuits. By contrast, in packet-basedsignals, the bit stream can be divided according to the destinationaddress of each packet.

Each gateway 100A, 100B can separate incoming TDM signals intoindividual DS0 signal streams. In one implementation, shown in FIG. 2,each gateway, such as the gateway 10A, includes a TDM switching matrix117 that provides full switching capabilities. The switching matrices117 permit the DS0 circuits to be interconnected flexibly withnarrowband channels appearing on the gateways. Echo cancellation andother digital signal processing functions can be performed in a digitalsignal processing portion 118 of each gateway. The signal processingportion 118 includes a pattern generator 122 and a pattern detector 124for generating and detecting specified patterns of bits, respectively.The pattern generator 122 and pattern detector 124 can be implemented,for example, using microprocessors, digital signal processors, or customapplication specific integrated circuits (ASICs). DS0 signal streams areadapted by an ATM adaptation layer 120 into ATM cells. Each cell isinserted through the ATM ports 21 into an ATM cell stream 135 thattraverses an ATM network 101. The gateways include a control section 119that controls overall operation of the gateway. In one implementation,the gateways 100A, 100B are implemented as Salix 7720. Class-IndependentSwitches available from Tellabs Operations, Inc.

As illustrated in FIG. 1, each gateway 100A, 100B is connected to arespective ATM end point switch 115. The connection between a gatewayand an ATM end point switch 115 and the connection between the ATM endpoint switch and the ATM network 101 are user-network interfaces (UNIs).Within the ATM network 101, there are a number of ATM switches 110 whichare interconnected by network-node interfaces (NNIs).

A call control network 126, which forms part of an existing telephonesystem, runs parallel to the voice network. The call control network 126primarily controls telephone switching equipment to connect theoriginating and terminating ends of a telephone call using SS7 messages.A call controller 120A, 120B is coupled to each gateway 100A, 100B andprovides an interface between the gateway and the call control network126. The exchange of call control signals allows the gateways 100A, 100Bto establish a connection through the ATM network 101 to enable thetransmission of narrowband traffic between the end points.

As shown in FIGS. 3 and 4, to establish a voice connection, a user atthe originating end dials 210 a telephone number. A connection isestablished through an originating TDM circuit switch in the circuitswitched network 102A, and the call controller 120A at the originatingend receives 215 an SS7 initial address message (IAM) 150. The callcontroller 120A routes 220 the call, in other words, it identifies acall controller 120B associated with a terminating DS0 circuit in thecircuit switched network 102B. The call controller 120A also determines225 whether a continuity check operation is to be performed as part ofthe call set up. A trade-off exists between the desire to perform acontinuity check during each call set up and the extra time and overheadassociated with performing continuity checks. As a result, typicallyonly a percentage of the call set-ups will include a continuity checkoperation. In one implementation, approximately 5–10% of call set-upswould include a continuity check operation.

Assuming that the call controller 120A determines that a continuitycheck operation is to be performed, the call controller sends 230 aconnection control message (CreateConn) 152 to the originating gateway100A to initiate a connection through the ATM network 101. TheCreateConn message 152 includes an indication that a sending-sidecontinuity check operation is requested. In response, the gateway 100Areserves 235 resources for the call and makes the pattern generator 122and pattern detector 124 available. Connections are set up between theadaptation layer 120 and the pattern generator 122 as well as thepattern detector 124. The pattern generator 122 repeatedly generates 240a specified bit pattern, and the detector 124 monitors 245 the incomingpacket stream (if any) for the same bit pattern.

The particular bit pattern used may depend on the application. Forexample, in one implementation, the pattern generator 122 is programmedto generate a bit pattern such that both binary values in each possiblebit position in the packet can be checked. Thus, the pattern generator122 can generate a sequence of two complementary 8-bit values. Other bitpatterns can be generated to allow various types of potential failuresto be detected and isolated. Thus, in some implementations, the patterngenerator 122 is programmed to generate a bit pattern comprising asequence of 256 values, in other words, a sequence of all possible 8-bitvalues. More generally, the pattern generator 122 can be programmed togenerate a sequence of all possible n-bit values, where n is the numberof bits in each byte.

Another exemplary pattern is illustrated in FIG. 6 and includes twenty8-bit values. A first byte includes only “1”s, whereas a second byteincludes only “0”s. The third through tenth bytes include a singlebinary “1” with adjacent bytes differing by shifting the binary “1”value from one bit position to an adjacent bit position. Similarly, theeleventh through the eighteenth bytes include a single binary “0” withadjacent bytes differing by shifting the binary “0” value from one bitposition to an adjacent bit position. The nineteenth byte includes analternating pattern of “1”s and “0”s. The twentieth byte includes theinverse pattern of the nineteenth byte. Other bit patterns may be useddepending on the specific errors to be detected. Known errordetection/correction techniques may be useful in selecting anappropriate bit pattern for a given application.

After making the pattern generator 122 and detector 124 available, thegateway 100A returns 250 an acknowledgement message (CreateAck) 154 thatincludes a connection descriptor. The connection descriptor includes anATM address for the gateway 100A as well as information that uniquelyidentifies the call. The information that uniquely identifies the callcan identify a connection-related resource such as the narrowbandcircuit (e.g., DS0 circuit) handling the call on the originating side.

Next, the call controller 120A sends 255 an IAM message 156 to theterminating call controller 120B. The message 156 includes theinformation contained in the connection descriptor as well as anindication that the continuity check operation is to be performed. Uponreceiving the IAM message 156, the terminating call controller 120Broutes 260 the call. In other words, the terminating call controller120B selects a TDM circuit on a particular gateway, such as the gateway100B, to handle the call. The call controller 120B then sends 265 aconnection control message (CreateConn) 158 to the terminating gateway100B. The CreateConn message 158 also includes the information containedin the connection descriptor. In addition, the message 158 includes anindication that the receiving-side of a continuity check operation isbeing requested.

In response, the terminating gateway 100B establishes 270 a packetdomain connection 126 with the originating gateway 100A through thepacket network 101, as shown in FIG. 5. As part of setting up the packetdomain connection, the information that uniquely identifies the call isforwarded through the packet switches 110, 115 until it is received bythe originating gateway 10A. That allows the originating gateway 100A toassociate the packet-domain connection with the TDM-domain connectionfor the call. The gateways 100A, 100B and ATM switches 110, 115negotiate the ATM routing headers that will be used between hops alongthe packet-domain connection.

The gateway 100B also sets up 275 a continuity check loop between theincoming and outgoing packet streams 128, 130 associated with the packetnetwork connection. FIG. 5 shows the loopback provided in the TDM-domainof the gateway 100B. More generally, however, the loopback can beprovided in either the TDM-domain or the packet-domain depending on thetype of failures the continuity check is intended to detect. A controlmessage (CreateAck) 178 is sent 280 by the terminating gateway 100B tothe terminating call controller 120B to acknowledge that thepacket-domain connection has been established for the call and that thecontinuity check loop has been set up. The bit pattern appears assuccessive TDM samples in the TDM domain, when the loopback is providedin the TDM domain, and appears within the cell stream in the ATM domain.

As the pattern generator 122 repeatedly generates the specified patternof bits, the pattern detector 124 monitors 245 the incoming bit patternsand determines 285 (FIG. 3A) whether the incoming pattern matches thebit pattern that was generated by the generator 122. If the packetconnection is properly established and if the adaptation functions inthe gateways 10A, 100B are operating properly, the specified bit patternwill be detected by the detector 124 after traversing the packetconnection 116 and the continuity check loop.

The pattern detector 122 can include a software or hardware timer 132that provides a timeout function. If the pattern detector 124 does notdetect the generated pattern within the time set by the timer 132, thecontinuity check fails. In that case, the gateway 100A signals 165 thecall controller 120A or a management system (not shown) to inform 290 itof the failure. The results of the continuity check operation can beused to determine the cause of the failure.

On the other hand, if the generated pattern is detected within the timeset by the timer, then the continuity check is successful. The gateway100A disconnects 295 the pattern generator 122 to allow a purgeoperation to be performed so that the pattern of bits is not forwardedto the TDM circuit handling the call. Once the detector 124 no longerdetects the pattern, the adaptation layer 116 in the originating gateway100A is connected 300 to the DS0 circuit that is handling the call.

If the continuity check is successful, the gateway 100A also notifies305 its call controller 120A that the pattern has been detected. Theoriginating call controller 120A sends 310 an SS7 message to theterminating call controller 120B informing it of the successfulcontinuity check. In response, the call controller 120B instructs theterminating gateway 100B to disconnect 315 the loopback between theincoming and outgoing packet streams. The terminating gateway 100B isthen reconfigured 320 to continue processing the call.

Continuity check packets also can be used as a coarse determination ofthe Cell Delay Variation in the packet network. In that case, thepattern of bits should be sent over at least several cells.

The foregoing continuity check operations can be used in systemsemploying “robbed” bit supervisory signaling as well as clear channeloperation. However, when the continuity check packets are used in asystem employing “robbed” bit supervisory signaling, the fact that thelow order bits of some frames are used for the supervisory signalingshould be accounted for. For example, the bits that are used forsupervisory signaling can simply be ignored for the purpose of thecontinuity checks.

As described above, different call controllers 120A, 120B are associatedwith the gateways 100A, 100B. However, in some cases, both theoriginating and terminating gateways 100A, 100B may share a common callcontroller, such as the call controller 120A. In that case, a techniquesimilar to that discussed above can be used with a single callcontroller performing the functions of both call controllers 120A, 120B.When the call controller 120A routes the call after receiving the IAMmessage 150, it selects the terminating TDM circuit switch and thecorresponding terminating gateway 100B to handle the call. Also, when asingle call controller 120A is involved, the IAM message 156 need not beused.

Although the foregoing implementations have been described with respectto ATM networks, circuit-switched traffic can be routed over otherpacket-domain networks, such as frame relay, Ethernet and InternetProtocol (IP) networks, as well.

Continuity check operations based on a pattern of digital bits are notlimited to systems under the control of SS7 signaling. In addition,continuity checks can be performed independently of call set-upprocesses. For example, in some cases, the pattern generator 122 cancontinuously generate a pattern over an existing packet connection. Thepattern detector 124 monitors the return signals and checks whether thespecified pattern is detected. The output of the pattern detector 124then can be read on demand. Such testing can be used, for example, aspart of a maintenance program to determine how often failures occur on aparticular packet connection and its associated gateways.

For situations in which a gateway has multiple adapters for handlingconversions between packet-based and TDM-based bearers, separate patterngenerators 122 and pattern detectors 124 can be provided for eachadapter. Alternatively, the specified pattern can be broadcast over themultiple ATM channels and/or connections.

In some implementations, the timer 132 can be incorporated into the callcontroller 120A. In that case, the call controller 120A would determinethat the continuity check had failed if the gateway 100A did not notifyit that the continuity check was successful within the specified time.Alternatively, the call controller 120A can be programmed to query thegateway 100A regarding the success of the continuity check if thegateway has not provided an indication prior to the specified timeelapsing. The success or failure of the continuity check would then bedetermined based on the gateway's response.

Various features of the system can be implemented in hardware, software,or a combination of hardware and software. For example, some aspects ofthe system can be implemented in computer programs executing onprogrammable computers. Each program can be implemented in a high levelprocedural or object-oriented programming language to communicate with acomputer system. Furthermore, each such computer program can be storedon a storage medium, such as read-only-memory (ROM) readable by ageneral or special purpose programmable computer, for configuring andoperating the computer when the storage medium is read by the computerto perform the functions described above.

Other implementations are within the scope of the claims.

1. A method of performing a continuity check operation comprising:sending a sequence of cells, each cell having a specified pattern ofbits, over a packet network connection through a first interface on thepacket network to a second interface on the packet network wherein thepattern of bits is generated by a narrowband-to-packet network adapterhaving a bit pattern generator; monitoring the first interface for areturn sequence of cells over the packet network connection; anddeciding whether the continuity check is successful based on whether apattern of bits of the return sequence of cells matches the specifiedpattern of bits of the sent sequence of cells.
 2. The method of claim 1including providing a loop between incoming and outgoing packet streamsassociated with the packet network connection.
 3. The method of claim 1including repeatedly sending the sequence of cells over the packetnetwork connection during the monitoring.
 4. The method of claim 1wherein the sequence of cells sent over the packet network connectionincludes a first byte all of whose bits are a first value and a secondbyte all of whose bits are a second different value.
 5. The method ofclaim 1 wherein the sequence of cells includes multiple bytes eachhaving multiple bits, wherein a single bit in each byte has a value thatdiffers from all other bits in the byte, and wherein the bit having thedifferent value is shifted by one position between adjacent bytes. 6.The method of claim 1 wherein the sequence of cells includes first andsecond bytes each of whose bits alternate in value, and wherein thevalue of the second byte is the complement of the value of the firstbyte.
 7. The method of claim 1 wherein the continuity check isconsidered a failure if the return sequence of cells is not detected atthe first interface during monitoring within a specified period.
 8. Amethod of performing a continuity check operation comprising: sending asequence of cells, each cell having a specified pattern of bits, over apacket network connection through a first interface on the packetnetwork to a second interface on the packet network; monitoring thefirst interface for a return sequence of cells of the pattern of bitsover the packet network connection; and deciding whether the continuitycheck is successful based on whether a pattern of bits of the returnsequence of cells matches the specified pattern of bits in the sentsequence of cells, wherein the continuity check is performed during aset-up process for a narrowband call over the packet network.
 9. Themethod of claim 8 wherein the call set-up process includes sendingSignaling System 7 messages.
 10. An apparatus configured to adaptcircuit-switched and packet-based bearers and configured to executecontinuity check operations, the apparatus comprising a bit patterngenerator and a bit pattern detector, wherein the pattern of bits isgenerated by a narrowband-to-packet network adapter having a bit patterngenerator, wherein the pattern generator is arranged to generate asequence of cells, each cell having a specified pattern of bits to besent over a packet network connection, and the bit pattern detector isarranged to monitor a return sequence of cells over the packet networkconnection, wherein the apparatus is configured to decide whether acontinuity check is successful based on whether a pattern of bits of thereturn sequence of cells matches the specified pattern of bits in thesent sequence of cells.
 11. The apparatus of claim 10 wherein the bitpattern generator is arranged to send the sequence of cells repeatedlyover the packet network connection.
 12. The apparatus of claim 10configured to perform the continuity check during a set-up process for anarrowband call over the packet network connection.
 13. A communicationssystem comprising: a packet network; and a first gateway to adapt anarrowband to the packet network coupled to a first interface on thepacket network and configured to execute continuity check operations,wherein the gateway includes a bit pattern generator and a bit patterndetector, wherein the pattern generator is arranged to generate asequence of cells, each cell having a specified pattern of bits to besent over a connection in the packet network, and the bit patterndetector is arranged to monitor a return sequence of cells over thepacket network connection, wherein the gateway is further configured todecide whether a continuity check is successful based on whether apattern of bits of the return sequence of cells matches the specifiedpattern of bits in the sent sequence of cells.
 14. The system of claim13 including a second gateway coupled to a second interface on thepacket network and configured to provide a loop between incoming andoutgoing packet streams associated with the packet network connection.15. The system of claim 13 wherein the bit pattern generator is arrangedto send the sequence of cells repeatedly over the packet networkconnection.
 16. A communications system comprising: a packet network;and a first gateway coupled to a first interface on the packet networkand configured to execute continuity check operations, wherein thegateway includes a bit pattern generator and a bit pattern detector,wherein the pattern generator is arranged to generate a sequence ofcells, each cell having a specified pattern of bits to be sent over aconnection in the packet network, and the bit pattern detector isarranged to monitor a return sequence of cells over the packet networkconnection, wherein the gateway is further configured to decide whethera continuity check is successful based on whether a pattern of bits ofthe return sequence of cells matches the specified pattern of bits inthe sent sequence of cells, wherein the gateway is configured to performthe continuity check during a set-up process for a narrowband call overthe packet network connection.
 17. An article comprising acomputer-readable storage medium including computer-executableinstructions for causing a computer system to: send a sequence of cells,each cell having a pattern of bits, over a packet network connectionthrough a first interface on a packet network to a second interface onthe packet network; monitor the first interface for a return sequence ofcells over the packet network connection; decide whether a continuitycheck is successful based on whether a pattern of bits of the returnsequence of cells matches the specified pattern of bits in the sentsequence of cells; and perform the continuity check during a set-upprocess for a narrowband call over a packet network.
 18. A method ofperforming a continuity check between a first gateway and a secondgateway of a packet-switched network, the method comprising: generatinga sequence of cells in a pattern generator of the first gateway, eachcell having a specified bit pattern, transmitting the sequence of cellsfrom the first gateway to the second gateway; establishing a continuitycheck loop for the transmitted sequence of cells at the second gateway;using a pattern detector at the first gateway to monitor for a returnedsequence of cells from the second gateway; and determining whether thereturned sequence of cells matches the transmitted sequence of cells.